Anaerobic biological treatment of waste waters containing sulphates enables the quantitative conversion into sulphide. If the sulphide formed can be converted into useful by-products which can be removed from the liquid, the salt load of waste waters containing sulphate can be decreased dramatically. Application of this technology is of great interest in those cases where sulphates and sulphites are emitted by waste waters resulting from industrial processes, flue gas treatment, leachates etc.
In FR-A-2 484 990 a method is disclosed for stripping of hydrogen sulphide in a by-pass of the anaerobic reactor using biogas that is circulated between a stripper and a desulphurizing apparatus. If however this method will be applied for alkaline waste waters, the pH of the reactor liquid will rise beyond optimum values (for methanogenesis as well as for H.sub.2 S stripping) due to release of carbon dioxide as a consequence of a selective hydrogen sulphide removal. The pH-value of the reactor/stripper circulation liquid will rise if carbon dioxide leaves the system. This will occur in case large amounts of carbon dioxide are removed via the desulphurising apparatus, leading to an important release of carbon dioxide in the stripper. In the publication cited, a release of carbon dioxide is described.